Step 1In a large, heavy-bottom pot, heat the condensed milk, milk, cream, vanilla beans, espresso grounds and espresso until the liquid scalds. Remove from heat.
Step 2Meanwhile, in a large bowl, whisk together the eggs, sugar, salt and cornstarch.
Step 3Slowly ladle 2 to 3 cups of the scalded milk mixture in with the eggs while whisking to temper. Pour the egg mixture back into the pot and heat over medium heat. Cook, stirring continuously, until the mixture thickens to a custard. Immediately remove from heat.
Step 4Strain the mixture, using a chinois, into a bowl containing the cubed butter and vanilla extract. Once the mixture is strained, use an immersion blender to blend the custard until it is very smooth. Cover the surface of the custard with plastic wrap to prevent a skin from forming and refrigerate the custard until chilled, preferably overnight. This makes a generous 2 quarts custard, which will keep up to 3 days, refrigerated.
Step 5To serve, dollop the custard into cups and serve with lightly sweetened whipped cream and top with a sprinkling of finely ground espresso.